Burst mode laser transmitters are widely used in gigabit passive optical networks (GPONs). A functional block diagram of a prior art burst mode laser transmitter 100 is depicted in FIG. 1. Transmitter 100 comprises: a laser driver 110 for receiving a transmit data TXD, a transmit enable signal TE, a bias current IBIAS, and a modulation current IMOD, and outputting an output current IO; a laser diode 120 for receiving the output current IO and emitting a light signal; a optical fiber 130 for receiving the light signal; a photodiode (MPD) 140 for receiving a part of the light signal from a back facet of the laser diode 120 and outputting a photodiode current IPD, an automatic power control (APC) block 150 for receiving the photodiode current IPD along with the transmit data TXD and the transmit enable signal TE and outputting the bias current IBIAS and the modulation current IMOD.
Throughout this disclosure, VDD denotes a power supply node. An exemplary timing diagram for the transmitter 100 is shown in the lower half of FIG. 1. When the TE signal is de-asserted, the transmitter 100 is disabled and the output current IO is nil; in this case, the light signal emitted by the laser diode 120 is also nil. When the TE signal is asserted, the transmitter 100 is enabled and the output current IO will be modulated by the transmit data TXD in accordance with the bias current IBIAS and the modulation current IMOD in the following manner: IO is equal to IBIAS if TXD is 0, else IO is equal to IBIAS+IMOD; in this case, the light signal emitted by the laser diode 120 will be modulated by the transmitter data TXD such that the light intensity is equal to a first level P0 when TXD is 0 or else the light intensity is equal to a second level P1, wherein P0 and P1 are related to IBIAS and IBIAS+IMOD, respectively, via a transfer characteristics of the laser diode 120 that is temperature dependent. When the transmitter 100 is enabled, the light signal being emitted by the laser diode 120 also illuminates the photodiode 140 and causes the photodiode 140 to transmit the photodiode current IPD such that IPD is related to the light intensity via a transfer characteristics of the photodiode 140.
Ideally, the photodiode current IPD is equal to a first current IO when the light intensity is of the first level P0, and equal to a second current I1 when the light intensity is of the second level P1. APC 150 receives the photodiode current IPD. When TXD is 0, APC 150 compares IPD with a first reference current IREF0; if IPD is greater than IREF0, it indicates IBIAS is too large and needs to be decreased; otherwise IBIAS is too small and needs to be increased. When TXD is 1, APC 150 compares IPD with a second reference current IREF1; if IPD is greater than IREF1, it indicates IBIAS+IMOD is too large and needs to be decreased, otherwise IBIAS+IMOD is too small and needs to be increased. IBIAS and IMOD are thus adjusted in a closed loop manner so as to make IPD equal to IREF0 when TXD is 0 and equal to IREF1 when TXD is 1. The two reference currents IREF0 and IREF1 are determined in accordance with a combination of the characteristics of laser diode 120, the characteristics of photodiode 140, and the temperature, such that when IPD is equal to IREF0, (IREF1) the light intensity is equal to a first (second) target level. In practice, however, photodiode 140 usually has a very high parasitic capacitance so that IPD does not always track the light intensity accurately. Instead, IPD tracks the light intensity accurately only after a string of successive “0” of “1” bits of TXD. In the particular example shown in FIG. 1, a practical IPD reaches the level I0 (I1) only after two successive “0” (“1”) bits of TXD. Also, the high parasitic capacitance of the photodiode 140 makes it challenging to implement a high speed comparison circuit (for comparing IPD with IREF0 or IREF1).
Accordingly, what is desired is method and apparatus for effective automatic power control for burst mode laser transmitter in the presence of high parasitic capacitance of the photodiode.